Despite a great deal of study, we still know surprisingly little about the nature of the factors which are responsible for the virulence of M. tuberculosis both in the mouse and in man. Infection studies carried out in aerogenically challenged mice indicate that the virulent H37Rv continues to grow within the lung even after clear evidence is seen of an effective immune response in the liver and spleen. On the other hand, the avirulent variant, H37Ra shows a little initial growth in the lung followed by a relatively rapid clearance of the organisms from both the lungs and spleen. The latter group of mice developed little or no acquired resistance to a subsequent virulent challenge. A number of BCG and H37Ra recombinant to bearing randomly selected DNA fragments from M. tuberculosis Erdman or H37Rv are selected by infecting normal mice with the recombinants and following the growth of the organisms in the lungs of C57BL/6 (Bcgs) and A/I (Bcgn) mice and comparing the resulting growth curves with those for M. tuberculosis H37Rv and H37Ra as positive and negative controls. Recombinants (kanamycin-resistant) present in the lungs after 3 months (at which time the H37Ra population is undetectable) will be tested for the presence of the ivg gene and for their ability to grow within mouse macrophages in vitro. Loganithemically growing cultures of selected recombinants in macrophages or in both will be examined for the presence of specific protein sensitive able to induce DTH responses in the footpads of tuberculous mice. Mice will be vaccinated with the selected recombinants and the resulting level of acquired resistance to an aerogenic challenge compared with that seen in BCG and H37Ra vaccinated controls.